1. Field of the Invention
This invention relates to a printing plate which enables printing without use of dampening water.
2. Description of the Prior Art
Heretofore, a number of proposals have been made on printing plates comprising a silicone rubber layer as an ink repellent layer and thus enabling printing without use of water. For example, U.S. Pat. No. 4,358,522 discloses a printing plate having a silicone rubber layer provided on a photosolubilizable photosensitive layer comprising a quinonediazide compound backed with an aluminium plate through a primer layer. Further, U.S. Pat. No. 4,342,820 discloses a printing plate having a silicone rubber layer provided on a photosensitive layer comprising a product of partial esterification of 1,2-naphthoquinone-2-diazide-5-sulfonyl chloride with a phenolic novolak resin crosslinked with a polyfunctional isocyanate compound. In such printing plates, it is a common practice to provide a primer layer between a substrate and a photosensitive layer for the purpose of preventing halation, ensuring the adhesion between the substrate and the photosensitive layer, binding the substrate through incorporation of a primer layer containing fine particles or the like.
Although the conventional printing plates having the above-mentioned structures are useful, they have the following problems.
(1) An increase in the thickness of the photosensitive layer leads to a poor developability. On the other hand, as the thickness is decreased, the light absorbance of the photosensitive layer is decreased, which brings about transmission of an exposure light through the photosensitive layer and reflection of the transmitted light on the substrate or primer layer in the case of longer imagewise exposure time, which brings about halation and finally leads to a poor dot reproduction. The addition of a light absorber such as an ultraviolet absorber to the primer layer or photosensitive layer is considered to be effective to solve this problem. However, such an expedient leads to no significant halation preventing effect, because it is difficult to match the absorption wavelength of the absorber with that of a quinonediazide group contained in the photosensitive layer.
(2) Since known quinonediazide compounds which have been used as the photosensitive material have a relatively low molecular weight, pinholes and, non-uniform coating, etc. tend to occur in the photosensitive layer when a photosensitive layer has a small thickness, which gives an adverse effect on the printing.
(3) The stabilization of the adhesion between the photosensitive layer and the silicone rubber layer is of importance in respect of qualities of the printing plate such as developability and scratch resistance of printing plates. However, since the adhesion between the photosensitive layer and the silicone rubber layer tends to be influenced by external conditions, qualities of the printing plate also tend to undergo changes.